Finally a Royal Icing you want to eat!
1 cup unsalted butter
1 cup granulated white sugar
1 teaspoon vanilla extract
1/2 teaspoon almond extract
2 teaspoons baking powder
1/2 teaspoon salt
3 cups flour
510 g powdered sugar, divided
2 1/2 ounces egg whites (1/4 cup; 70g), from 2 large eggs
2 teaspoons rum or water
1 1/2 teaspoons vanilla extract
1/2 teaspoon kosher salt
1/4 teaspoon cream of tartar
3 tablespoons heavy cream (more for thinning royal icing, if desired)
Gel paste food coloring, optional
Preheat oven to 350° F. In the bowl of your mixer cream butter and sugar until smooth, at least 3 minutes. Beat in extracts and egg.
In a separate bowl combine baking powder and salt with flour and add a little at a time to the wet ingredients. The dough will be very stiff. If it becomes too stiff for your mixer turn out the dough onto a countertop surface. Wet your hands and finish off kneading the dough by hand.
DO NOT CHILL THE DOUGH. Divide into workable batches, roll out onto a floured surface and cut. You want these cookies to be on the thicker side (closer to 1/4 inch rather than 1/8).
Bake at 350 for 6-8 minutes. Let cool on the cookie sheet until firm enough to transfer to a cooling rack.
Measure 450g of powdered sugar and place in the bowl of a stand mixer, then add egg whites, rum, vanilla, salt, and cream of tartar. Stir with a flexible spatula to form a smooth paste, then set over a gently simmering water bath in a 3-quart sauce pan and stir until paste is hot to the touch, or about 150°F. Transfer to stand mixer fitted with a paddle attachment, add remaining 60g of powdered sugar and mix at low speed to combine. Increase speed to medium and beat until thick and frosting-like, about 20 seconds or until the mixing bowl feels cool to the touch.
To thin the icing, reduce speed to low, add cream, and continue mixing until smooth. Remove bowl from mixer and lift paddle attachment, moving it in a figure 8 pattern as icing streams back into bowl. If the figure 8 disappears in exactly 8 seconds, the icing is ready to use as a cookie glaze. If it melts away faster, thicken by adding a spoonful of powdered sugar. If it takes longer than 8 seconds to disappear, thin by adding a few drops of cream. Repeat this test, making adjustments as you go, until the figure 8 takes exactly 8 seconds to disappear.
To Color the Icing, if Desired: Divide icing among several small bowls, one for each color you’d like to prepare. Cover each with plastic wrap pressed directly against the surface. Working with one bowl at a time, stir in a few drops of gel paste. When your desired color has been achieved, immediately transfer to a disposable pastry bag. Frost as desired.
Cookies frosted with royal icing can be stored up to 1 week at room temperature; in a pinch, their shelf life can be extended to about 10 days with refrigeration.